Medical diagnosis, gene analysis, and the like are required to be conducted with promptness, high efficiency, and simplicity. Therefore recently, importance is given to a technology allowing highly-sensitive detection of a biogenic substance in very small amounts.
For example, surface plasmon resonance is used in a method for detecting biogenic substances such as proteins, hormones and low molecular weight compounds contained in test solutions of blood, perspiration, urine, or the like. Surface plasmon resonance occurs by free electrons in a metal interacting with electromagnetic waves (light). A detection method using surface plasmon resonance does not require labeling of the biogenic substance and is simple, differing from a fluorescence detection method and an electrochemical method.
Examples of surface plasmon resonance include propagating surface plasmon resonance and localized surface plasmon resonance.
A sensor using propagating surface plasmon resonance has, for example, a triangular prism. A thin metallic film is formed on one of the faces of the triangular prism. From a different face of the prism, light is applied to the face having the thin metallic film. When light enters into the thin metallic film from a certain angle, propagating surface plasmon resonance occurs. This certain angle is referred to as the resonance angle. The resonance angle depends on the refractive index (permittivity) of a material that is present near the thin metallic film (about 100 nm). Therefore, the propagating surface plasmon resonance sensor can highly sensitively detect changes in property of a nearby material.
When propagating surface plasmon resonance is used in a biosensor, antibodies are immobilized on the surface of the thin metallic film. By bringing a test solution containing biogenic substances (antigens) into contact with the surface of the thin metallic film, the antigen and the antibody react with and bond to each other. Since the refractive index near the thin metallic film changes, the resonance angle changes. If a correlation is obtained in advance between the concentration of antigens contained in the test solution and the resonance angle, the antigen concentration can be calculated from the change in the resonance angle.
Patent Literature 1 discloses a method that uses localized surface plasmon resonance. According to Patent Literature 1, dielectric particulates coated with noble metal (hereinafter, referred to as “particulates”) are formed in a plurality of regions of the substrate surface. Antibodies that bond to biomolecules are immobilized on particulates formed in one region (hereinafter, referred to as “signal region”). The signal region indicates a resonant wavelength reflecting the concentration of the biomolecules. Particulates formed in another region (hereinafter, “reference region”) are treated such that bonding to biomolecules is prevented from occurring. By irradiation with light, localized surface plasmon resonance is observed in each region. In the signal region, bonding to the biomolecules changes a reflectance spectrum. Meanwhile, in the reference region, biomolecules do not bond, and thus a reflectance spectrum does not change. An amount of reflectance spectrum change caused by bonding to the biomolecules can be obtained on the basis of the difference between the reflectance spectrum in the signal region and the reflectance spectrum in the reference region. The concentration of the biomolecules is calculated on the basis of the amount of the reflectance spectrum change.